Because gaseous fuels such as natural gas, propane, hydrogen, and blends thereof are cleaner burning fuels compared to liquid fuels such as diesel, recent attention has been directed to developing engines that can burn such fuels while matching the power and performance that engine operators are accustomed to expecting from diesel engines.
Natural gas fuelled engines that use lean-burn spark-ignition (“LBSI”) introduce the fuel into the intake air manifold or intake ports at relatively low pressures. To avoid engine knock caused by the premature detonation of the fuel inside the combustion chamber, such engines typically operate with a compression ratio no greater than about 12:1, which is lower compared to diesel-cycle engines which have compression ratios of at least 14:1, and this affects engine performance and efficiency. Consequently, while the exhaust gases from the combustion chambers of LBSI engines can have lower emissions of NOx, and PM compared to an equivalently sized diesel engine, such LBSI engines also have lower performance and energy efficiency, which means that to do the same amount of work, more fuel is consumed on an energy basis, and to match the full range of power and performance of a diesel engine, a larger LBSI engine is needed.
Recently, research has been directed towards blending natural gas and hydrogen for use in homogeneous charge, spark-ignition engines. Representative publications relating to such research include, “The Effects of Hydrogen Addition On Natural Gas Engine Operation”, SAE Technical Paper 932775, by M. R. Swain, M. J. Yusuf, Z. Dulger and M. N. Swain, which was published by the Society of Automotive Engineers (“SAE”) in 1993; “Variable Composition Hydrogen/Natural Gas Mixtures for Increased Engine Efficiency and Decreased Emissions”, ASME Journal of Engineering for Gas Turbines and Power, Vol. 122, pp. 135-140, by R. Sierens and E. Rousseel, published in 2000; “Hydrogen Blended Natural Gas Operation of a Heavy Duty Turbocharged Lean Burn Spark Ignition Engine”, SAE Technical Paper 2004-01-2956, by S. R. Munshi, C. Nedelcu, J. Harris, et al., published in 2004; “Hydrogen Enrichment: A Way to Maintain Combustion Stability in a Natural Gas Fuelled Engine with Exhaust Gas Recirculation, the Potential of Fuel Reforming”, Proceedings of the Institution of Mechanical Engineers, Part D. Vol. 215 2001, pp. 405-418, by S. Allenby, W-C. Chang, A. Megaritis and M. L. Wyszynski; “Emission Results from the New Development of a Dedicated Hydrogen-Enriched Natural Gas Heavy-Duty Engine”, SAE Technical Paper 2005-010235, by K. Collier, N. Mulligan, D. Shin, and S. Brandon which was published in 2005; “Comparisons of Emissions and Efficiency of a Turbocharged Lean-Burn Natural Gas and Hythane-Fuelled Engine”, ASME Journal of Engineering for Gas Turbines and Power, Vol. 119, 1997, pp. 218-226, by J. F. Larsen and J. S. Wallace; “Effect of hydrogen addition on the performance of methane-fuelled vehicles. Part I: effect on S.I. engine performance”, International Journal of Hydrogen Energy, Vol. 26. 2001, pp. 55-70, by C G. Bauer and T. W. Forest; “Methane-Hydrogen Mixtures as Fuels”, International Journal of Hydrogen Energy, Vol. 21 No. 7, 1996, pp. 625-631, by G. A. Karim, I. Wierzba and Y. Al-Alousi; and “Internal Combustion Engines Fuelled by Natural Gas-Hydrogen Mixtures”, International Journal of Hydrogen Energy, Vol. 29, 2004, pp. 1527-1539, by S. O. Akansu, Z. Dulger, N. Kahraman and T. Veziroglu. The results reported in these papers have shown that at stoichiometric operation, the addition of hydrogen tends to reduce power density and increase NOx, while slightly reducing hydrocarbon and carbon monoxide emissions. A more significant effect is reported under lean premixed conditions, where a substantial increase in the lean limit is observed. This has been attributed to enhanced combustion rate and shorter ignition delay. For a given air-fuel ratio, NOx emissions are higher with hydrogen addition, due to the higher flame temperature, while CO and unburned hydrocarbons are substantially reduced. However, due to hydrogen's ability to extend the lean limit, lower NOx emissions can be achieved by running at leaner air-fuel ratios with hydrogen addition. Flame stability in the presence of exhaust gas recirculation (EGR) is also improved. Efficiency effects can depend upon the tested operating condition, with some studies such as those reported in the Swain, Sierens, and Akansu papers, showing improved efficiency with hydrogen addition and other studies, such as those reported in the Larsen and Bauer papers, showing reduced efficiency. Such contradictory results show that while a considerable amount of research has been done to investigate the effects of blending natural gas and hydrogen for use in homogeneous charge spark-ignition engines, the combustion process is complex, that the effect of combusting such fuel mixtures in an engine can be very dependent upon the engine operating conditions, and that the effect of adding hydrogen and the magnitude or such effects, if any, are not obvious or easy to predict. Furthermore, all of the published papers referenced herein relate to homogeneous charge spark-ignition engines, and while some laboratory experiments have been reported, such as shock-tube studies and non-premixed counterflow methane/heated air jet experiments, the inventors are not aware of any publications relating to experiments involving fuelling a direct injection internal combustion engine with a blended fuel mixture comprising methane and hydrogen.
Engines that are capable of injecting a gaseous fuel directly into the combustion chamber of a high compression internal combustion engine are being developed, but are not yet commercially available. Engines fuelled with natural gas that use this approach can substantially match the power, performance and efficiency characteristics of a diesel engine, but with lower emissions of NOx, unburned hydrocarbons, and PM. NOx are key components in the formation of photochemical smog, as well as being a contributor to acid rain. PM emissions, among other detrimental health effects, have been linked to increased cardiovascular mortality rates and impaired lung development in children. However, with direct injection engines that are fuelled with natural gas, it has been found that there is a trade-off between NOx emissions and emissions of unburned hydrocarbons and PM. That is, later timing for injecting the natural gas is beneficial for reducing NOx but results in higher emissions of unburned hydrocarbons and PM. Environmental regulatory bodies in North America and around the world have legislated substantial reductions in NOx and PM emissions from internal combustion engines. As a result, because it is necessary to reduce the emissions of each one of NOx, PM and unburned hydrocarbons, for a direct injection engine fuelled with natural gas, the higher PM emissions associated with later combustion timing effectively limits how much the timing for fuel injection can be retarded.
Since published technical papers have reported that under specific operating conditions there can be benefits arising from fuelling a homogeneous charge, spark-ignition engine with a gaseous fuel mixture comprising methane and hydrogen, and since environmental regulatory bodies have legislated substantial reductions in NOx and PM emissions from internal combustion engines, and since the combustion process is complex and the effect of adding hydrogen to a fuel mixture delivered to a direct injection internal combustion engine is unpredictable, there is a need to determine whether it is possible to improve combustion stability and reduce engine emissions by fuelling a direct injection internal combustion engine with hydrogen and natural gas, and if so, the method of operating a direct injection engine that is fuelled with such fuels to achieve improvements in combustion stability and reductions in engine emissions.